1,2-Benzisothiazol-3-one compounds are useful as antimicrobial agents, antifungal agents, etc. Patent Literature 1 listed below discloses a production method therefore comprising reacting a 2-(alkylthio)benzonitrile compound with a halogenating agent in the presence of water. In this method, after mixing a 2-(alkylthio)benzonitrile compound with water, a halogenating agent is added thereto and then reacted. This method achieves a relatively high yield, but there is room for further improvement.